Well.. because they actually have the best concept artists - the number of rewards they've got is impressive, I don't think there is a studio with better artists atm.

My wife is dealing with graphics and said that they couldn't get better team together. She is dealing with web design/brand identification now, but she has quite a few concept pieces done some time ago (especially in our "WoW period" ;] ). All this GW2 hype is making her nostalgic and maybe I can make her to paint something again =].